Some sensors are used to detect plural physical quantities by detecting changes in output signals. Such sensors include, for example, a pressure sensor having a Wheatstone bridge, which is formed of gauge resistors (piezoelectric resistors) arranged on a diaphragm movable with pressure and the like. This pressure sensor outputs an output potential difference, which is variable with a quantity of deformation of the diaphragm caused by pressure, by using a change in a resistance value caused by deformation of the gauge resistors. It is thus possible to measure pressure applied to the pressure sensor by detecting the output potential difference.
The resistance value of a sensor circuit including the Wheatstone bridge also varies with temperature. That is, it is possible to measure temperature of an environment, in which the pressure sensor is located, based on the change in the resistance value of the Wheatstone bridge forming the pressure sensor.
In an exemplary system disclosed in U.S. Pat. No. 8,701,460 (patent document 1), when pressure is measured, an output potential difference is measured by supplying a resistor-forming part of a Wheatstone bridge with a predetermined power voltage and pressure is detected based on the output potential difference. On the other hand, when temperature is measured, an on-chip resistor is inserted between a power supply and the resistor-forming part and temperature is detected based on a potential at an intermediate node between the on-chip resistor and the resistor-forming part.
In the exemplary system disclosed in patent document 1, since a total resistance value varies between time of pressure measurement and time of temperature measurement and hence a current consumption of the power supply varies, the voltage of the power supply tends to vary among measurement time.
U.S. Pat. No. 7,483,765 (patent document 2) discloses an exemplary sensor, which is driven not to cause variation of current consumption. Although this sensor also includes a resistor element, which corresponds to an on-chip resistor used in patent document 1, the resistor element is not controlled to be present and absent. Alternatively, an input to an A/D converter is switched over between the pressure measurement time and the temperature measurement time. Thus variation of current consumption is suppressed.
In the sensor disclosed in patent document 2, however, the resistor element is connected in series to a resistor-forming part of a Wheatstone bridge. As a result, in comparison to the sensor driving system disclosed in patent document 1, a driving voltage of the sensor decreases and sensitivity in pressure detection decreases. When a resistance value of the resistor element is decreased to increase the sensitivity in pressure detection, a difference in input signal levels to the A/D converter increases between the pressure measurement time and the temperature measurement time. That is, a dynamic range of the input to the A/D converter need be increased. As a result, the A/D converter and subsequent circuits, which follow the A/D converter, need be large-sized and increase costs. En addition, the input level at the temperature measurement time becomes close to a power supply potential or a reference potential. As a result, leak currents of a multiplexer and switches, which are used to switch over the input signals to the A/D converter between a pressure signal and a temperature signal, tend to increase.